One of the common themes in the previous pages is the carbon dioxide, CO2, in our atmosphere and oceans. CO2 is it is one of the major drivers of long-term atmosphere and ocean changes on our Earth through time and is often mentioned when discussing climate change. Many people, for good reason, misunderstand why increasing amounts of CO2 in our atmosphere and oceans are harmful. Commonly, scientists hear “CO2 was much higher at other times in Earth’s history, so the current increase in CO2 is the Earth’s natural variation”. The first part of this statement is correct: CO2, at times, was much higher than today in the geologic past. The other part of the sentence “the current increase in CO2 is the Earth’s natural variation” is not correct. Human activity on Earth has increased the amount of CO2 entering the atmosphere and oceans, altering the natural cycling of CO2 in the Earth’s systems (oceans, atmosphere, rock, and biosphere). From here
The giant caveat: Earth system sensitivity describes climate changes over hundreds of thousands of years, not the decades and centuries that are immediately relevant to humans. The authors say that over long periods, increases in temperature may emerge from intertwined Earth processes that go beyond the immediate greenhouse effect created by CO2 in the air. These include melting of polar ice sheets, which would reduce the Earth’s ability to reflect solar energy; changes in terrestrial plant cover; and changes in clouds and atmospheric aerosols that could either heighten or lower temperatures.
The researchers confirmed the long-held belief that the hottest period was about 50 million years ago, when CO2 spiked to as much as 1,600 ppm, and temperatures were as much as 12 degrees C higher than today. But by around 34 million years ago, CO2 had dropped enough that the present-day Antarctic ice sheet began developing. With some ups and downs, this was followed by a further long-term CO2 decline, during which the ancestors of many modern-day plants and animals evolved. This suggests, the paper’s authors say, that variations in CO2 affect not only climate, but ecosystems.
The new assessment says that about 16 million years ago was the last time CO2 was consistently higher than now, at about 480 ppm; and by 14 million years ago it had sunk to today’s human-induced level of 420 ppm. The decline continued, and by about 2.5 million years ago, CO2 reached about 270 or 280 ppm, kicking off a series of ice ages. It was at or below that when modern humans came into being about 400,000 years ago, and persisted there until we started messing with the atmosphere on a grand scale about 250 years ago.
Of modern ages:
In the late 1700s, the air contained about 280 parts per million (ppm) of CO2. We are now up to 420 ppm, an increase of about 50%; by the end of the century, we could reach 600 ppm or more. As a result, we are already somewhere along the uncertain warming curve, with a rise of about 1.2 degrees C (2.2 degrees F) since the late 19th century.
Another reason carbon dioxide is important in the Earth system is that it dissolves into the ocean like the fizz in a can of soda. It reacts with water molecules, producing carbonic acid and lowering the ocean's pH (raising its acidity). Since the start of the Industrial Revolution, the pH of the ocean's surface waters has dropped from 8.21 to 8.10. This drop in pH is called ocean acidification, and it interferes with the ability of marine life to extract calcium from sea water to build skeletons and shells.
A timelapse here of global emissions
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